Doom was just the nail. Economically, Commodore was messed up by the time Doom came around. But I agree, Doom made it very evident, I remember it well - suddenly, a 486 became a "must have" for rich geeks.
I grew up with a Commodore 64 but once my older brother started getting loaner machines from work, it was quickly dispatched.
We were usually playing Pirates!, Carmen Sandiego, Cauldron I & II, Ultimate Wizard & the Bard's Tale games and dialing into BBS on the C64. The usual way of dialing with leaked calling card numbers.
Eye of the Beholder on the 386SX machines was cool and all, but once he 'liberated' a 486DX from work and we started playing Doom and using his job's dialup service, the C64 was done.